Quick Answer
The correct answer is that the table is partitioned but the partition is not correctly defined, which is the most likely reason an Athena query returns only one row from a large CSV file. When a table’s partition mapping does not match the actual folder structure or partition keys in Amazon S3, Athena scans only the metadata of the mismatched partition rather than the full dataset, effectively reading just a single row or summary entry. On the AWS Certified Machine Learning Specialty MLS-C01 exam, this scenario tests your understanding of how Athena’s partition pruning works and how a WHERE clause on a partition column—like a date—can fail silently if the partition definition is misaligned with the S3 prefix. A common trap is assuming a 1 MB CSV must contain many rows, but the real issue is that the query never touches the data. Memory tip: “Partition mismatch prunes your data to a single stitch.”
This MLS-C01 practice question tests your understanding of exploratory data analysis. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A data scientist queried an Athena table and got only one row back, but the CSV file is 1 MB. What is the most likely reason?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
The table is partitioned but the partition is not correctly defined
Correct: If date partition is not correctly mapped, the filter may return no data.
The CSV file contains only one row
Why wrong: Incorrect: 1 MB file typically contains many rows.
The table is not an external table
Why wrong: Incorrect: External tables are standard for Athena.
Athena does not support CSV format
Why wrong: Incorrect: Athena supports CSV.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
The table is partitioned but the partition is not correctly defined
Option B is correct because the file is large but the query returned only one row, suggesting the table's partition mapping is wrong; the WHERE clause on date may not match the actual partition. Option A is wrong because if the table were external, it would still read all data. Option C is wrong because 1 MB file likely has many rows. Option D is wrong because Athena supports CSV.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
The table is partitioned but the partition is not correctly defined
Why this is correct
Correct: If date partition is not correctly mapped, the filter may return no data.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
The CSV file contains only one row
Why it's wrong here
Incorrect: 1 MB file typically contains many rows.
The table is not an external table
Why it's wrong here
Incorrect: External tables are standard for Athena.
Athena does not support CSV format
Why it's wrong here
Incorrect: Athena supports CSV.
Common exam traps
Many certification questions include familiar terms but test a specific constraint. Read the exact wording before choosing an answer that is generally true but wrong for this case.
Detailed technical explanation
This question should be treated as a scenario, not a definition check. Identify the problem, the constraint and the best action. Then compare each option against those facts.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A cloud solutions architect for a retail company is evaluating services for a new workload. The correct answer here reflects best practice for the specific scenario described — not a general cloud recommendation. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Cloud exam questions reward reading the constraint carefully: the same technology can be right or wrong depending on the use case.
What to study next
Identify which MLS-C01 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
Exploratory Data Analysis — study guide chapter
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FAQ
Exploratory Data Analysis — This question tests Exploratory Data Analysis — Read the scenario before looking for a memorised answer..
The correct answer is: The table is partitioned but the partition is not correctly defined — Option B is correct because the file is large but the query returned only one row, suggesting the table's partition mapping is wrong; the WHERE clause on date may not match the actual partition. Option A is wrong because if the table were external, it would still read all data. Option C is wrong because 1 MB file likely has many rows. Option D is wrong because Athena supports CSV.
Identify which MLS-C01 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 20, 2026
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